Skin acts as a natural barrier between internal and external environments and therefore plays an important role in vital biological functions such as protection against mechanical and chemical injury, micro-organisms, and ultraviolet damage. The health and appearance of skin, however, can deteriorate due to environmental factors, genetic makeup, nutrition, and sun exposure.
The negative effects of exposure to ultraviolet (“UV”) light are well-known. Prolonged exposure to sunlight causes damage such as sunburn to the skin and dries out hair making it brittle. When skin is exposed to UV light having a wavelength of from about 290 nm to about 400 nm, long term damage can lead to serious conditions such as skin cancer.
UV light also contributes to aging by causing free radicals to form in the skin. Free radicals include, for example, singlet oxygen, hydroxyl radicals, the superoxide anion, nitric oxide and hydrogen radicals. Free radicals attack DNA, membrane lipids and proteins, generating carbon radicals. These in turn react with oxygen to produce a peroxyl radical that can attack adjacent fatty acids to generate new carbon radicals. This cascade leads to a chain reaction producing lipid peroxidation products. Damage to cell membranes results in loss of cell permeability, increased intercellular ionic concentration, and decreased ability to excrete or detoxify waste products. The end result is a loss of skin elasticity and the appearance of wrinkles. This process is commonly referred to as photo-aging.
Although UV radiation can be problematic, over 90% of solar energy is from visible light and infrared (IR) radiation. Visible light and IR radiation, especially IRA (770-1400 nm), penetrates deep into human skin than UV radiation, and can easily reach the dermis. Visible light, especially high energy visible light, can induce pigmentation in a similar manner as UVA, and IR radiation can impair proper functioning of mitochondria. Visible light and IR radiation both contribute to photoaging by degrading collagen through induction of MMP1 enzyme (without induction of the natural endogenous protection MMP1 inhibitor), generating free radicals and decreasing antioxidant content in human skin.
With aging, the outer skin layer (epidermis) thins, even though the number of cell layers remains unchanged. The number of pigment-containing cells (melanocytes), however, decreases. Therefore, the skin appears pale and translucent. Large pigmented spots (age spots, liver spots, or lentigos) may appear in sun-exposed areas. Changes in the connective tissue reduce the skin's strength and elasticity. It is more noticeable in sun-exposed areas (solar elastosis). Elastosis produces the leathery, weather-beaten appearance common to farmers, sailors, and others who spend a large amount of time outdoors. Dehydration increases the risk of skin injury. Poor nutrition can also negatively impact the skin, causing dryness, rash, and puffiness.